My qmail setup has worked quite well, though I have had a bit of irritating thing with it. After opening a number of folders in Thuderbird, it stopped responding. After reading and seraching a bit I discovered it was a configuration issue in the courier-imap server.
Probably didnt read it too well when installing but QmailRocks mentioned one should increase MAXPERIP if running in a small home or office network.
Now it works as a sharm.